<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>position + margin</title>
    <style>
        .father {
            width: 300px;
            height: 300px;
            background-color: blue;
            position: relative;
        }
        /* 需要知道子元素的高度，margin值要设置具体值 */
        .son {
            width: 100px;
            height: 100px;
            background-color: pink;
            position: absolute;
            top: 50%;
            margin-top: -50px;

        }
    </style>
</head>
<body>
    <div class="father">
        <div class="son"></div>
    </div>
    
</body>
</html>